Garbage Collector doubt

i have little confusing understanding the concept of garbage collector. according to my understanding from k&B, object gets eligible for garbage collection when that object is no more reachable.
we can explicitly make objects eligible for GC either by nulling a reference, reassigning a reference variable or by isolating a reference variable.

builds a house, and writes the address of that house on gc1. Then it builds a second house, and writes that address on gc2.

GarbageCollection gc3;

puts a blank address card in your rolodex, and says "this is where I will write down the address for gc3".

gc1=gc2;

This erases the old address on gc1, and replaces it with the address on gc2. Note that nothing about gc2 changes. You now have two variables with the same address. No card holds the address of the first thing you built. You have no way to find it - it is now eligible for garbage collection.

your doStuff() method does indeed return the address of a newly created object. You can call doStuff and ignore what it returns, which means you could create an object that is immediately eligible for gc, but this case, you do save it in your gc3 variable.

There are only two hard things in computer science: cache invalidation, naming things, and off-by-one errors